PPDL Picture of the Week for
August 6, 2012

Herbicide Carryover in Dry Conditions

Travis Legleiter, Weed Science Professional Assistant, Botany & Plant Pathology, Purdue University

Many herbicides applied to corn and soybean acres are desired because of their ability to persist in the soil following application and thus control emerging weeds into the growing season.  Not all herbicides persist in the soil; some are broken down almost immediately upon soil contact while others can persist for several months.  The duration of herbicide persistence in soil is dependent on how quickly the herbicide is broken down into its parent materials either through biotic or abiotic processes.  The rate of herbicide degradation processes is largely dependent on soil moisture and temperature.  While we have had plenty of warm soil temperatures, the lack of moisture in many regions of the state has likely slowed the degradation of many herbicides applied this growing season.  The amount of precipitation to be received between now and next spring is still to be seen, but the likelihood of some herbicide carryover into next year is high at this point in time.

Two herbicides that are likely to carryover into the next cropping season due to dry conditions are atrazine and imazethapyr.  Atrazine applied to corn acres this year could potentially injury early planted soybeans next spring, while imazethapyr that was applied postemerge in soybeans this year may injure corn next spring.
Atrazine carryover is most likely if split applications were made at maximum rates onto high pH soils. Symptoms of atrazine carryover on soybean is  necrosis of the margins of cotyledons, interveinal chlorosis on the first true leaves, and necrosis beginning at the leaf tips of the newest leaves.  Severe carryover can cause significant soybean stand losses.

The carryover of imazethapyr is likely in fields where it was applied postemergence this season and under soil conditions of low pH, high OM, and heavy clay soils.  Symptoms of imazethapyr carryover on corn include stunting, interveinal chlorosis, pruned lateral roots, and bottle brushing of roots.  The majority of corn plants can grow out of imazethapyr injury, but severe cases can cause stand losses. 

There are several other corn and soybean herbicides that also have the potential to carryover into next season because of the dry conditions.  Although, all in all the amount of carryover is still dependent on how much precipitation we receive between now and next spring.

Click image to enlarge

Soybean seedlings with chlorotic interveinal tissue and necrotic leaf tip and cotyledon margins due to Atrazine carryover.

Bottle brushed roots of a corn seedling due to carryover of Imazethapyr applied postemerge in soybean the season prior.

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service